The configuration and amplitude of the QRS complex vary considerably among individuals. The duration is usually between 0.06 and 0.10 second. An abnormally prolonged QRS complex may indicate a block in the normal conduction pathways through the ventricles (such as a block of the left or right bundle branch). During the ST interval, the entire ventricular myocardium is depolarized. Therefore, the ST segment normally lies on the isoelectric line. Any appreciable deviation of the ST segment from the isoelectric line may indicate ischemic damage to the myocardium. The QT interval, sometimes referred to as the period of “electrical systole” of the ventricles, is closely correlated with the mean action potential duration of the ventricular myocytes. The duration of the QT interval is approximately 0.4 second, but it varies inversely with the heart rate, mainly because the duration of the myocardial cell’s action potential varies inversely with the heart rate (see
Fig. 16.16
In most leads, the T wave is deflected in the same direction from the isoelectric line as the major component of the QRS complex, although biphasic (i.e., oppositely directed) T waves are perfectly normal in certain leads. Deviation of the T wave and QRS complex in the same direction from the isoelectric line indicates that the repolarization process is proceeding in a direction counter to that of the depolarization process. T waves that are abnormal either in direction or in amplitude may indicate myocardial damage, electrolyte disturbances, or cardiac hypertrophy.
The original ECG lead system was devised by Willem Einthoven at the beginning of the 20th century. In this system, the vector sum of all cardiac electrical activity at any moment is called the resultant cardiac vector. This directional electrical force is considered to lie in the center of an equilateral triangle whose apices are located in the left and right shoulders and the pubic region (
Fig. 16.27 ). This triangle, called Einthoven’s triangle, is oriented in the frontal plane of the body. Hence, only the projection of the resultant cardiac vector on the frontal plane is detected by this system of leads. For convenience, the electrodes are connected to the right and left forearms rather than to the corresponding shoulders because the arms represent simple electrical extensions of leads from the shoulders. Similarly, the leg represents an extension of the lead system from the pubis, and thus the third electrode is generally connected to an ankle (usually the left one).
Certain conventions dictate the manner in which these standard limb leads are connected to the electrocardiograph. Lead I records the potential difference between the left arm and the right arm. The connections are such that when the •Fig. 16.27 Einthoven’striangle,illustratingtheelectrocardiographicconnectionsforstandardlimbleadsI,II,andIII.
potential at the left arm (VLA) exceeds the potential at the right arm (VRA), the tracing is deflected upward from the isoelectric line. In
Fig. 16.27
Fig. 16.28 , this arrangement of connections for lead I is designated by a plus sign at the left arm and by a minus sign at the right arm. Lead II records the potential difference between the right arm and the left leg, and the tracing is deflected upward when the potential at the left leg (VLL) exceeds VRA. Finally, lead III registers the potential difference between the left arm and the left leg, and the tracing is deflected upward when VLL exceeds VLA. These connections were chosen arbitrarily so that the QRS complexes are upright in all three standard limb leads in most normal individuals.
If the frontal projection of a resultant cardiac vector at some moment is represented by an arrow (tail negative, head positive), as in
Fig. 16.27 , the potential difference, VLA − VRA, recorded in lead I is represented by the component of the vector projected along the horizontal line between the left arm and the right arm, also shown in
Fig. 16.27 . If the vector makes an angle (θ) of 60 degrees with the horizontal line (as in